ARM: dts: omap3-overo: Add default trigger for TWL4030 LED

Commit c971ff18

 'leds: leds-pwm: Defer led_pwm_set() if PWM can sleep'
fixed a crash when using a trigger with a pwm-led provided by an
external chip. Now it is safe to add the default trigger according
to board-overo.c.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>